The present invention relates to the use of data communications equipment (DCE) in a leased line environment.
Some data communications networks have connections to both a leased line and a dial-up line between data communications equipment. Typically, the leased line is the primary communications channel for communicating information between the two modems while the dial-up line, i.e., the secondary communications channel, provides a backup capability when communications over the leased line is interrupted. Usually, this interruption is not a complete break in the data connection between the two modems over the leased line but rather is represented by a degradation in the signal quality of the data signal communicated over the leased line which results in an increase in the error rate. When this increase in error rate is detected by one of the two modems, it terminates data communications over the primary channel and automatically establishes a data connection to the other modem over the secondary channel, i.e., the dial-up line, through the public switched telephone network (PSTN). However, it is preferable to switch back to the leased line as soon as possible since the leased line usually costs less than the dial-up line. Currently, there are a number of procedures available for use in a modem for testing the leased line's effect on a signal so that the modem may estimate the line quality of the leased line and, if possible, disconnect the dial-up connection and switch back to using the leased line.
In one method, the modem interrupts the dial-up connection for a period of time and attempts to perform a modem train over the leased line with the other modem. If modem training is unsuccessful then the modem assumes that the leased line channel is still impaired and resumes data communications over the dial-up connection. However, when modem training is successful, communications is restored over the leased line, and the dial-up connection is dropped.
In another method, the modem creates a test signal and transmits this signal on the leased line while transmitting the data signal over the dial-up connection. In one embodiment of this method the modem creates the test signal using a slow speed modulation scheme, e.g., CCIT V.21, over the leased path while still sending the data signal using a high speed modulation scheme, e.g., CCITT V.32bis, over the dial-up connection. In this embodiment, the modem does not switch back to the leased line until the number of errors detected by the modem on the received test signal is less than a predetermined amount. Another embodiment of this method is illustrated by U.S. Pat. No. 5,138,609 issued Aug. 11, 1992 to Hashimoto. In this patent, the test signal created by the modem is a single frequency signal, i.e., a tone, that is transmitted over the leased line. In this embodiment the modem does not switch back to the lease line until the modem successfully detects the single tone.
Unfortunately, the above-mentioned prior art, while providing methods to test the leased line, are not complete answers to the problem. For example, the first method interrupts the data communications over the dial-up connection while testing the leased line, and the second method adds the complexity of creating a test signal in the modem, which does not provide for more sophisticated analysis of the primary channel.